1. Field of the Invention
This invention relates to amplifier circuits, in general, and to linear, low noise amplifier circuits in particular, especially those with high third order intercept points.
2. Prior Art
There are many amplifier circuits known in the art. For the most part, amplifier circuits are developed to provide a maximum output signal in response to an input signal. In some cases, the output signal maximum is related to the power generated by the amplifier. In other cases, the signal maximum is related to the amplitude of the voltage or the current at the output of the amplifier.
In many applications, it is desirable to reduce the noise generated by the system through the expedient of appropriate matching. This frequently results in an optimum noise operation but with a sacrifice in gain of the amplifier circuit.
On the other hand, if the amplifier is used in conjunction with a system which is arranged to observe or detect a small signal in an environment of large signals, the small signal can often be lost. That is, if the amplifier is used to limit or filter the larger signal, the smaller signal is also limited and is frequently lost.
To avoid this loss, some systems have been devised using compressive receiver techniques or the like. This technique permits a small signal to be picked out of signal pattern including larger signals. However, this procedure and apparatus tends to be quite complex and costly.
Another approach is to use an isolator, a low noise amplifier and another isolator (arranged in series) to perform this function. However, the isolators frequently have a tendency to be rather lossy wherein this technique is less than perfect.
Thus, a relatively simple, inexpensive technique for separating low level signals from a large scale signal background is highly desirable.